In facilities, such as a melting furnace and a heating furnace, in which the internal temperature reaches high temperatures of 400° C. or more, the inner walls of facilities include cooling panels or refractories. The inner walls of facilities emphasizing suppression of heat dissipation include refractories.
Regarding the lining structure of the inner wall of a melting furnace, a heating furnace, and the like, for example, a molten-steel ladle is provided with a wear refractory on the working face serving as the heating surface of refractories to be in contact with molten steel. The face of the wear refractory opposite the working face is called a rear face. A permanent lining refractory is provided on the rear face side of the wear refractory. The side of the permanent lining refractory opposite the wear refractory is further provided with an outer steel shell.
When there is a concern about damage due to thermal shock caused by a large temperature change, when there is a concern about erosion by a reactive gas or a molten product, or when a refractory is used for applications in a reducing atmosphere, carbon, such as graphite or carbon black, or a carbon compound, such as silicon carbide, pitch, or phenol resin, is typically added to the refractory at 1 mass % or more as a measure to improve the durability of the refractory. In this specification, a refractory containing such carbon or such a carbon compound is generally called a carbon-containing refractory. Adding carbon and/or a carbon compound to a refractory can improve the durability of the refractory by taking advantage of the high thermal conductivity of carbon and the carbon compound, a large angle of contact with oxide melt, a high filling property, and the binder effect on a refractory component.
When a carbon-containing refractory is used, the rear face is oxidized and an oxidized layer is formed on the rear face side of the carbon-containing refractory. Hereinafter, oxidation on the rear face side is called rear face oxidation, and the oxidized layer formed on the rear face side is called a rear-face oxidized layer. The working face of the refractory wears due to contact with slag on the working face side of the refractory. At this time, in the refractory containing carbon or a carbon compound, carbon or the carbon compound is oxidized and eliminated even inside the rear face and, as a result, the porosity of the oxidized layer increases according to the amount of carbon decreased. When wearing proceeds and the working face of the refractory reaches the oxidized layer on the rear face side, the refractory wears rapidly, which may cause a problem.
In contrast, in Patent Literature 1, a measure has been taken by generating a difference between the proportion of an antioxidant on the working face side and that on the rear face side of the carbon-containing refractory, namely, changing the composition of a refractory in the lining thickness direction. In Patent Literature 2, there has been proposed a method for inhibiting rear face oxidation of a carbon-containing refractory. The method includes placing a refractory protection sheet formed of a metal plate between an outer steel shell and the refractory.